1. Field of the Invention
This invention relates to a system for transmission of signals to or between underwater installations, comprising optical fibres for the transmission of signals to/from a control unit or between the installations.
2. Description of the Related Art
In modern offshore oil installations several underwater installations are presently often used as processing plants and wells. In addition to be supplied with power these installations will also have to be supplied with control signals.
In signal transmission over longer distances optical fibres have proved to be a favourable solution. However, in transmission and distribution of signals between installations within a limited area optical transmission represents a very expensive solution, as there are stringent requirements on accuracy as well as water resistance of the optical fibres and because the coupling processes performed under water can be very complicated. Locally there will be an advantage to have other coupling types and solutions.
A number of proposals exist for solving the problems of underwater fibre optic coupling. One example is given in U.S. Pat. No. 5,007,697 concerning an inductive coupler where the optical signal is converted to an electrical signal in the first coupling part and is transferred via induction to the other coupling part before the signal is converted back to an optical signal. This solution represents one example of the complex and expensive solutions of the prior art, and comprises, in addition to the coupling part itself, inductive coupling arrangements for taking power from existing power lines.
A system for transmission of signals in a combined RF and optical network is described in U.S. Pat. No. 5,892,773. In this case optical fibre is coupled to a conversion unit which is also coupled to one or more cables. The conversion unit converts optical signals to electrical signals in the radiofrequency range which is further distributed in the network using among other things control of the frequency ranges of the electrical signals. This system may in principle be used in underwater applications as there is a prerequisite for only a single fibre optic coupling, however, this requires dedicated cables for the further transmission of the signal. These cables either have to transmit the signals on to every single installation directly or have to be branched out using additional coupling points. In addition a power supply and corresponding equipment for operating the conversion unit are required, and the solution maintains the need for a dedicated network for the signal transmission, which increases the complexity of the underwater installation. This solution is for this reason not very attractive for underwater installations either.